Microsoft has addressed sign-in and email delivery problems affecting some on-premises Exchange Server systems after installation of the October 2026 Windows Server security updates.
The company acknowledged the issue in mid-October. Users on systems running Windows Server 2019 and later could have trouble accessing email or signing in to Outlook. Messages sent from Outlook could remain in the Outbox, and Exchange could have trouble accessing mailboxes during mailbox replication.
Microsoft said the problems stem from a new default limit on the memory Endpoint Mapper can use when handling Endpoint Lookup calls. Their severity and onset vary with server load, and some servers may not be affected. Microsoft also said the issue is separate from the October 2026 Exchange Server security updates and advised customers not to uninstall those updates.
In an update to its Windows release health dashboard, Microsoft said the November 2026 Patch Tuesday updates, KB5131080 and KB5131090, address the issue. It advised affected users to install them on all affected systems.
Microsoft also provided a temporary workaround for administrators who cannot deploy the cumulative updates immediately. It involves changing a registry key on affected Exchange Server systems to raise the Endpoint Lookup call limit and Endpoint Mapper memory limit, then restarting the system. Administrators should monitor the Exchange environment after the reboot and raise the memory limit if the issue returns.
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